Routers are very complex and diverse devices. We support a lot of them. Odds are, if PFConfig can not log into your router, you have simply picked the wrong router from the long list of supported routers.
Routers have software in them called Firmware. When manufacturers create a new firmware for a router, they sometimes change things inside the router, among them the way that port forwarding is done. We have a huge database of different firmwares for every router. If you do not know what firmware your router uses, you can pick the first router in the list that matches your router's model number and try it. If it doesn't work, try the next router in the list.
Some routers, like the WRT54G series have over 10 different firmwares to try.
Another possibility is that you are trying to use the wrong username and password for your router. This is different than your PFConfig username and registration code. If in doubt, try to log into your router manually until you figure out which username and password that you need to use. PFConfig will not break into your router or crack your username and password for you. You must know these 2 pieces of information.